Method of and bath for anodic treatment of aluminum



Patented Oct. 23, 1934 METHOD OF AND BATH FOR ANODIC TREATMENT OFALUMINUM Robert w. Buzzard, Kensington, Md. No Drawing. Applicationlgiay 22, 1934,

Serial No. 726,8 7

8 Claims.

(Granted under the act of March 3, '1883, as

amended April 30, 1928; 370 0. G. 757) This invention relates to amethod of preparing a bath for forming oxide coatings on aluminum or itsalloys by anodic treatment and the method of using the same. It is theobject of this invention to provide a bath that may .be used over agreater range of temperature, voltage and chromium concentration in thebath than in the previous practice and which may be used in a continuousprocess treatment of the [0 material. I

l This application is a continuation in part of my copending applicationSerial No. 668,419.

I have found that more satisfactory operation of baths of the typespecified may be had by using both hexavalent and trivalent chormium inthe solution. The addition of oxalic acid to a solution of chromic acidresults in the reduction of some of the hexavalent ions of chromic acidto the trivalent condition. It is also possible to produce trivalentchormium in such baths by treatment with any dibasic or polybasicorganic acid or salts of such acid, in particular the chromium salts,although the other salts will produce the effect.- Care has 5 to betaken that foreign elements are not so introduced which will bedetrimental to the bath. Due to ease of addition, low cost for a highpurity product, and ease of obtaining, oxalic acid has been mostgenerally used. In addition, more satisfactory films for certainpurposes result if a chromate, for example, potassium or sodiumdichromate, is added to the electrolyte.

Heretofore the test results have been secured when the maximum ratio ofoxalic acid to chromic acid is not greater thangone. However, by aproper adjustment of temperature and voltage conditions it is probablethat proportions in excess of that specified will be found suitable.

The application of potential may be varied widely, for example, thevoltage may be raised to the maximum as rapidly as the source of powerwill allow or as slowly as .the operator may desire. A good film will beformed in one hour but it may be applied in less time or over a longerperiod if conditions are such as to make it preferable. It has beenfound that a'change in temperature will greatly afiect the currentdensity of the bath. A rise in temperature causes a corresponding risein the current density. However, no definite relation has as yet beendetermined between the temperature and the current density. Further, itis known that changes in temperature influence the final properties ofthe film and its rate of formation, etc. These limits are dependent onthe composition of the bath and it is therefore not possible to setdefinite limits or ranges.

The voltage also influences the character of the film. It appears thateach definite solution has an optimum voltage at which a good film isformed at the lowest current density. This is not always the best film'for corrosion properties but is usually very close to the highest usablevoltage for best results.

As examples of the solutions that may be used the following are given:

3% chromic acid and 1% oxalic acide, bothby weight, using a constantvoltage of 40 volts with the bath at 40 C., the time of applicationbeing about one hour.

The bath may be made u with 3% chromic acid, 1% oxalic acid, and 1%potassium dichromate, all by weight, the voltage, time and tem- 75perature being as given in the preceding example.

The bath may be made up of 5% chromic acid and 4% oxalic acid, such abath operating very satisfactorily at 35 volts and 30 C. for one hour.Good films may be formed at other cycles.

The bath may advantageously be made up of 10% chromic acid and 5% oxalicacid, making practicable a broad range of temperature and voltagecombinations with greatly lowered current densities. I

It must be borne in mind that when oxalic acid and chromic acid reactthat varied results may be obtained and the actual. resultant isdiificult to predict. It has been found that it is best, therefore, tostate a concentration range for oxalic acid rather than a definitepercentage for the production of a desired result. Such as that adefinite current-density may be obtained in a 10% bath by adding 3-5%oxalic acid, as test runs will indicate.

In general, the percentage of total chromium may vary from 2% to 30%,wherein the trivalent chromium is regulated from 5% to 60% of the 100total chromium to properly adjust the bath and with 1% to 25% 01' achromate.

Owing to the fact that the oxalic acid is by the reaction with chromicacid converted into carbon dioxide and water, there is very little vorno organic residue left in the bath if there is sufiicient chromic acidpresent. It has been found that baths prepared according to thisinvention produce very good oxide films at lower current densities thanin the commercial processes now in use. Further, it permits of a verywide range of chromium concentration, voltage, temperature and time oftreatment. It is to be noted that, theoretically, no oxalic acid remainsin the bath as such. It has further been noted that there is a tendencyfor the lowering of the efiective temperature and voltage ranges whenoxalic acid is added to these baths. This tendency is more marked as theratio of oxalic acid to chromic acid in the bath is increased; also thistendency is more marked as the chromic acid concentration of thesolution is increased.

The bath may be prepared by making up a solution having the desiredconcentration 01 chromic acid and then adjusting the ratio of hexavalentchromium to trivalent chromium by adding the oxalic acid in the tankitself and heating by the method ordinarily employed to bring the bathin the tank to the desired temperature, which will initiate the reactionbetween the chromic acid and the oxalic acid. The oxalic acid will reactin a cold solution but the reaction is greatly hastened by theapplication oi heat to the bath. 1

If found preferably for any reason, the chromic acid may be reduced in aseparate container and added to the anodizing bath. One mole of oxalicacid should reduce approximately twothirds of a mole of chromic acid tothe trivalent condition, depending on the efficiency of the reaction.Little heat is required to start the reaction and if the concentrationof chromic acid is sufficient it is unnecessary to apply heat; forexample, when 1% oxalic acid by weight is added to 2.81% chromic acidsolution at the operating temperature. of the straight chromic acid baththe reaction proceeds of itself and produces about 0.35% trivalentchromium in the solution., It is desired to point out that heretofore ithas been deemed necessary to avoid trivalent chromium in anodizingbaths, whereas in the practice of the present invention trivalentchromium is intentionally formed in, or introduced into, theelectrolyte.

An intensive study of the usual straight chromic acid baths showed thatthere is little or no accumulation of trivalent chromium in the bath,very definitely not in the proportions as recommended for the trivalentbaths, and that the failure of such baths is due to the accumulation ofother elements than trivalent chromium. Results of tests show that thelife of the trivalent baths appears to bear a direct ratio to the totalchromium content.

A bath made up as above set forth and used as described is contraryptothe heretofore recognized peculiarities of such. baths and methods, inthat they operate at constant voltage, have no particular cycles, goodfilms may be applied in 30 minutes to 60 minutes, and the surfacecharacteristics of the film may be changed by varying the operatingtemperatures or voltages of the bath. The addition of oxalic acid to'the 'bathcauses the formation of good films with lower voltage andtemperature ranges and more satisfactory current densities.

The fact that it is not necessary to change the voltage or thetemperature during the treatment of an article makes it possible toplace atticles in the bath for treatment at any time and remove themwhen a satisfactory film has been formed thereon regardless of the timeof placing in or taking out of any other articles, that is, the bath maybe 'used in a continuous process.

It is sometimes desirable to permit the solution adhering to the articlewhen removed from the bath to dry thereon as there is thus provided amore eifective corrosion resistant coating for some purposes. Anysolution remaining in cracks or crevices does not attack the metal andwhen the solution is once thorough dry on the metal it is extremelydiflicult to remove.

The invention described herein may be manufactured and used by or forthe Government of the United Statesof America for governmental purposeswithout the payment of any royalty thereon.

I claim:

1. A bath for the anodic treatment of aluminum or its alloys containingboth hexavalent and trivalent chromium and a chromate wherein the totalchromium is from 2% to 30%, by

a dibasic or polybasic organic acid, the total chromium in the finalsolution being between 2% and 30% and the quantity of said product addedbeing sufficient to supply up to 60% 01 the total chromium.

3. In a method of preparing a bath for anodio treatment of aluminum orits alloys, the steps of causing substantially oxalic acid to react withchromic acid, and adding to a bath containing chromic acid a sufficientquantity of the product of said reaction to make the total chromium anydesired mrcentage up to 30% and thechromium added in said product beingup to 60% of the total chromium.

4. A bath for the anodic treatment of aluminum or its alloys containingboth hexavalent and trivalent chromium ions wherein the total chromiumis from 2% to 30% and the trivalent the step of passing current throughsuch material as an anode in a chromic acid bath containing 2% to 30%total chromium, whereof from 5% to 60% of the total chromium is in thetrivalent condition and which may also contain 1% to 25% of a chromate,said method being adapted to be practiced as a continuous process.

6.'A method of anodically treating material that is predominantly oialuminum, comprising the step otpassing a potential at a constantvoltage between 10 volts and 50 volts through such material as an anodein a bath containing 2% to 30% total chromium, whereof from 5% to 60% ofthe total chromium is in the trivalent condition and which may alsocontain 1% to 25% of 'a chromate, said bath being kept at a constanttemperature between 20 C. and 60' C.,

andsa.idmethodbeingadapt ed'tobemctiood asacontinuousprooess.

7.Amethodofanod1callytreatingmterlal predominantly of aluminum,comprising the stepsotpassingcurrentthmughmaterlflu'manodeinabathcontain1n32%fiol0%total chromium, whereof fifom 5% to60% oftotal chromium is in the trivalent condition and may 1% to25%otachromamsaid p method being adantedtobepmctioedu m I no

